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Radius of Curvature and Rectification

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Radius of Curvature and Rectification Study material of B.Sc.(Semester - I) US01CMTH02 (Radius of Curvature and Rectification) Prepared by Nilesh Y. Patel Head,Mathematics Department,V.P.and R.P.T.P.Science College US01CMTH02 UNIT-2 1. Curvature Let f : I ! R be a sufficiently many times differentiable function on an interval I. Then the points on the graph of y = f(x) is a curve. However, not all curves could be represented as a graph of such a real valued function on intervals viz, the figure of a circle with centre (0,0) and radius 1 in the XY -plane R2 is one such example of a curve. In this situation, we have to represent the equation of the circle as x = cos t; y = sin t, t 2 [0; 2π]. These are called the parametric equations of the circle. Also, let us think of a spring put in R3. Then the points of this spring is a curve. Thus formally we have the following definition of a curve. 1.1. Definition. Let I be a closed interval and x = x(t), y = y(t) and z = z(t) be real- valued differentiable functions defined on I. Then the points (x(t); y(t); z(t)) in the space is called a locus of the curve represented by the parametric equations x = x(t), y = y(t), z = z(t), t 2 I. Throughout this chapter we shall be concerned only with curves lying in the XY - plane. For such curves we have z = 0. Hence they are described by x = x(t) and y = y(t). A curve lying only in one plane is called a planer curve. 1.2. Definition. Let x = x(t); y = y(t) be a curve. If we eliminate t and obtain a relation g(x; y) = 0, then this form is called the cartesian representation of the curve. Further, if g(x; y) = 0 can be written in the form y = f(x) (respectively, x = f(y)), then y = f(x) (respectively, x = f(y)) is called the cartesian equation of the curve. 1.3. Example. Let I = [0; 1] and x = t, y = t2. Then this is a curve that can also be represented by the cartesian equation y = x2. 1.4. Definition. Let y = f(x) be a curve. Fix a point A on this curve. For a point P on the curve, let s = arc AP be the arc length from A to P . For a point Q on the curve, 1 2 let s + ∆s = arc AQ so that ∆s = arc PQ. Let `1; `2 be the tangents to the curve at the points P and Q making angles and + ∆ respectively, with a fixed line in the plane. Y " Clearly, the angle between these two tangents is ∆ , called the total bending or total curvature of the arc be- `2 tween P and Q. Hence the `1 Q ∆ average bending or the av- ∆s erage curvature of the curve s A P between these two points + ∆ ! relative to the arc length is O# X ∆ given by ∆s . The bending or Figure 1.4 the curvature of the curve at P is defined to be d = lim ∆ = lim ∆ . ds Q!P ∆s ∆s!0 ∆s 2. Derivative of an arc 2.1. Proposition. Fix a point A(x0; y0) on a curve given by y = f(x). For a point P (x; f(x)) on the curve, let s be the arc length of arc AP . (Clearly, s is a function of x:) Then prove that s ( ) ds dy 2 = 1 + : dx dx 2. Derivative of an arc 3 Proof. Let y = f(x) represent the Y " Q given curve and A be a fixed point on it. Let P (x; y) be a generic point on the curve. ∆y Let the arc AP = s. Take A ∆s s P a point Q(x + ∆x; y + ∆y) N on the curve near to P . ∆x Let arc AQ = s + ∆s. From the right angled trian- gle 4PNQ, we have, ! O# L M X Figure 2.1 PQ2 = PN 2 + NQ2 = (∆x)2 + (∆y)2 ( ) ( ) PQ 2 ∆y 2 ) = 1 + ∆x ∆x ( ) ( ) ( ) chord PQ 2 ∆s 2 ∆y 2 ) = 1 + : arc PQ ∆x ∆x Taking Q ! P , we get chord PQ ! arc PQ. Hence, ( ) ( ) s ( ) ds 2 dy 2 ds dy 2 = 1 + ) = 1 + : dx dx dx dx The proof of the following corollary is left to the reader. 2.2. Corollary. Let x = x(t) and y = y(t) be the parametric equations of a curve. Then s( ) ( ) ds dx 2 dy 2 = + : dt dt dt 2.3. Exercise. In the Proposition 2.1, suppose that the curve is represented by x = f(y). r ( ) 2 ds dx Then deduce that the derivative of the arc length dy = 1 + dy . 2.4. Definition. Let h(x; y) = 0 be a cartesian representation of a curve. By substituting x = r cos θ and y = r sin θ, in this form we get a representation g(r; θ) = 0 of the curve called a polar representation of the curve. We shall be mainly dealing with the form r = f(θ) of the curve. 2.5. Theorem. For a polar equation r = f(θ) of a curve, s ( ) ds dr 2 = r2 + : dθ dθ 4 Proof. Let r = f(θ) represent the given curve and A be a fixed point on it. Let P (r; θ) be a generic Y " point on the curve. Let Q(r + ∆r; θ + ∆θ) N s arc AP = s. Take a ∆ point Q(r + ∆r; θ + ∆θ) r P (r; θ) on the curve near to P . + ∆ r s Let arc AQ = s + ∆s. r A From the right angled trian- gle 4ONP as shown in fig- θ ∆ ure, we have, # θ ! O X Figure 2.5 PN PN sin ∆θ = = ) PN = r sin ∆θ OP r and ON ON cos ∆θ = = ) ON = r cos ∆θ: OP r Also, from the figure, NQ = OQ − ON = r + ∆r − r cos ∆θ = r(1 − cos ∆θ) + ∆r ∆θ = 2r sin2 + ∆r: 2 Now from the right angled triangle 4PNQ, we have, PQ2 = PN 2 + NQ2 ∆θ )PQ2 = r2 sin2 ∆θ + (2r sin2 + ∆r)2 "2 ! # ( ) ( ) ( ) ( ) 2 PQ 2 sin ∆θ 2 ∆θ sin ∆θ ∆r ) = r2 + r sin 2 + ∆θ ∆θ 2 ∆θ ∆θ ( ) ( ) ( ) 2 chord PQ 2 arc PQ 2 sin ∆θ 2 ) = r2 arc PQ ∆θ ∆θ " ! # ( ) ( ) 2 ∆θ sin ∆θ ∆r + r sin 2 + : 2 ∆θ ∆θ 2 Taking Q ! P , we get chord PQ ! arc PQ. Hence, ( ) ( ) s ( ) ds 2 dr 2 ds dr 2 = r2 + ) = r2 + : dθ dθ dθ dθ 3. Radius of curvature 5 2.6. Example. For the curve rn = an cos nθ, prove that ds n−1 = a(sec nθ) n : dθ Solution. Here rn = an cos nθ. Taking log on both the sides, n log r = n log a + log(cos nθ): By differentiating this we get, n dr sin nθ dr = −n ) r = = −r tan nθ r dθ cos nθ 1 dθ ) 2 2 2 2 2 2 r + r1 = r (1 + tan nθ) = r sec nθ s ( ) ds dr 2 ) = r2 + dθ dθ 1 n−1 = r sec nθ = a(cos nθ) n sec nθ = a(sec nθ) n : 2.7. Exercise. 1. Show that curvature of a circle is constant and is equal to the reciprocal of its radius. 2. Show that curvature of a straight line is zero. ds 3. Find dx for the following curves. ( ) (i) y = a cosh x . a2 a (ii) y = a log a2−x2 . ds 4. Find dt for the following curves. (i) x = a(t − sin t); y = a(1 − cos t). (ii) x = a(cos t + t sin t); y = a(sin t − t cos t). (iii) x = aet sin t; y = aet cos t. ds 5. Find dθ for the following curves. (i) r = a(1 − cos θ). (ii) r2 = a2 cos 2θ. 3. Radius of curvature 3.1. Definition. Let P be a point on a curve such that the curvature of the curve at P is nonzero. Then the radius of the curvature at P is defined to be the reciprocal of the ds curvature at P and is denoted by ρ. That is, ρ = d . 3.2. Theorem. Let y = f(x) be a curve and P be a point on it. Then prove that the radius of curvature at P is given by 3 (1 + y2) 2 ρ = 1 ; y2 dy d2y where y1 = dx and y2 = dx2 . 6 Proof. dy Let y = f(x) be the given curve. Then tan = dx . Differentiating with respect to s, we get, ( ) ( ) d d dy d dy dx dx sec2 = = = y ds ds dx dx dx ds 2 ds d dx )(1 + tan2 ) = y ds 2 ds d dx )(1 + y2) = y 1 ds 2 ds ds 1 + y2 ds )ρ = = 1 d y dx q 2 1 + y2 ) 1 2 ρ = 1 + y1 y2 (1 + y2)3=2 )ρ = 1 : y2 3.3. Theorem. Let r = f(θ) be a polar form of a curve with a point P on it. Then prove that the radius of curvature at P is given by (r2 + r2)3=2 ρ = 1 ; 2 2 − r + 2r1 rr2 0 00 where r1 = f (θ) and r2 = f (θ). Proof. From the figure it is clear that = θ + '. Hence, d dθ d' = + ds ds ds dθ d' dθ = + ds ( dθ ds ) dθ d' = 1 + : (3.3.1) ' ds dθ P (r; θ) r We know that tan ' = .
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